A hang-on-back filter is a type of aquarium filter that is designed to be mounted on the back of a fish tank. It works by drawing water from the tank, filtering it through various media, and then returning the clean water back into the aquarium. This type of filter is popular for its ease of installation and maintenance, making it suitable for both beginners and experienced aquarists. These filters typically use a combination of mechanical, chemical, and biological filtration methods. Mechanical filtration removes debris and particles, while chemical filtration can eliminate toxins and odors. Biological filtration supports beneficial bacteria that break down harmful substances, contributing to a healthier environment for fish and other aquatic life.